Carbon nanofiber growth on thin rhodium layers

A thinlayer of carbon nanofibers (CNFs) was synthesized on a thin polycrystalline rhodium (Rh) metal layer by decomposing ethylene in the presence of hydrogen. Interaction of Rh crystals with carbon results in fragmentation and formation of Rh-nanoparticles, facilitating CNF growth. CNFs are immobilized on the surface by an apparently amorphous intermediate layer containing both Rh and C. Maximum CNF growth was achieved at intermediate hydrogen concentrations. A CNF growth mechanism by (sub-) surface diffusion of carbon on Rh is suggested. Carbon nanofibers/tubes (CNF/Ts) are finding applications in e.g. electronic devices, electrochemical capacitors, catalyst support layers in microreactors, additives to polymers and super hydrophobic coatings. Controlled growth of these structures is important to realize application in nanoscale devices. Synthesis of CNF/Ts by catalytic chemical vapor deposition method offers high rate of carbongrowth and easy control of the reaction conditions [1]. Fe, Co, Ni are widely studied metals for CNF/T growth, however, the presence of residual ferro-magnetic catalyst particles in the CNF/Ts may be an obstacle for research on the magnetic properties of CNF/Ts, as well as for applications in electronic devices, electrochemical capacitors and catalysts. In addition to ferro-magnetic metals, also Pd, Pt, Ag, Au, Ru, Re, Rh, Ir and Os enable CNF/T growth. So far, exclusively Pd was studied for growing CNF/Ts on thin metal films. In the present work, we report on synthesis CNFs on thin Rh layers, exploring the effect of the conditions on CNF growth. A 100 nm polycrystalline rhodiumlayer was deposited at room temperature on a single crystalline silicon wafer (Ø 100 mm) using DC-sputtering technique. Prior to rhodium deposition, a 15 nm thick titanium adhesion layer was sputter deposited on the silicon wafer. CNF formation was carried out at atmospheric pressure in a quartz reactor. The fresh wafers were heated in nitrogen (100 ml/min) from room temperature to 600 °C (6 °C/min), followed by reduction in H2 (5 vol.%) for 15 min. After flushing with N2 for 5 min, ethylene was introduced, growing CNFs during 30 min. The concentration of ethylene (10 vol.%) in the feed was kept constant in all experiments, whereas the hydrogen concentration was varied between 0 and 20 vol.% (balance N2, total flowrate 100 ml/min). Pretreatment of fresh wafers with H2 results in significant sintering of Rh grains from ∼10 nm to ∼85 nm, as can be seen in Fig. 1. XRD analysis of the wafers confirmed this as the intensity of the Rh(1 1 1) peak-width decreased significantly (Table 1). The only phase in the layer was metallic Rh according to XRD. Carbon deposition on the Rh surface resulted in the formation of CNFs with metal particles in the tips (Fig. 2), containing exclusively Rh and carbon, according TEM–EDX. This confirms that Rh particles are responsible for the formation of CNFs and no other metals contribute to the growth